Here's a rough idea of what I originally envisaged the GUI looking like, basically along the lines of MAMEUI which is where this PC screenshot came from.

In this example its showing one of the snapshots and the description file (description files are in the JFDI spec, but none of the ZIP's currently contain them. Refer to the ZIP file structure here), we should also have tabs to show other things such as the cover art or accompanying instructions/manuals/protection etc.

Initially, we should start with the list on the left and add the right section for snapshots later, once we have a PNG module.

If PNG code exists for C, that would be an obvious choice to go for although does lead to compatibility issues with CLib and RO version. We are currently dependent on CLib for ZLib, although I do plan to go back and remove that dependency at some point.

BASIC works across all versions of RO and has no dependencies, although we obviously have to write a converter for PNG to Sprites - which can't be that hard...can it? I've yet to look into it, although it sounds like some of the links you've provided may well provide source.